Communication network and method of wireless communication

ABSTRACT

A communication network, a method of determining at least one propagation characteristic, and a method of wireless communication are disclosed. The communication network comprises a plurality of aerial vehicles that each supports at least one respective directional antenna, and a processing element for determining at least one propagation characteristic for each respective wireless communication channel between at least one user equipment and each aerial vehicle of the plurality of aerial vehicles.

PRIORITY CLAIM TO RELATED APPLICATIONS

This application is a U.S. national stage filing under 35 U.S.C. § 371from International Application No. PCT/GB2019/053162, filed on 8 Nov.2019, and published as WO2020/095056 on 14 May 2020, which claims thebenefit under 35 U.S.C. 119 to United Kingdom Application No. 1818289.9,filed on 9 Nov. 2018, the benefit of priority of each of which isclaimed herein, and which applications and publication are herebyincorporated herein by reference in their entirety.

The present invention relates to a communication network and to a methodof wireless communication between user equipment and aerial vehicles. Inparticular, but not exclusively, the present invention relates to theprovision of high speed broadband services from high altitude platforms(HAPs) in which an indication of channel attenuation and phase shift isdetermined for each wireless communication channel between userequipment and HAPs in a network to thereby enable co-operativebeamforming to be effectively used to create a multi-channel wirelesscommunication link.

The provision of wireless communication using aerial vehicles has beensuggested for many years. Various types of aerial vehicles such astethered balloons or manned aircraft or unmanned aircraft have beensuggested. High-altitude platforms (HAPs) have been suggested as a typeof aerial vehicle. In particular the provision of high speed broadbandservices from HAPs has now been discussed in the prior art for a numberof years. Various authors have discussed how HAPs deployed in thestratosphere around 15 to 22 km in altitude, can achieve an excellenttrade off between terrestrial cellular networks and satellite basedsystems. HAPs have the advantage that they are capable of coveringsignificantly wider areas with Line-of-Sight (LoS) communication linkscompared with terrestrial systems yet do not suffer from capacity andpropagation delay limitations typically provided by satellite basedsystems. For example, geostationary satellites are located approximately1800× further from the earth's surface than HAPs.

Wireless networks are required to deliver high aggregate data rateswithin a limited bandwidth through efficient spectral use. A way thiscan be achieved is to use directional antennas on each HAP. For example,it has been suggested that user equipment such as mobile phones, tabletsor laptops or other such user devices can communicate wirelessly with anumber of HAPs equipped with directional antennas such as horn ormulti-element phased array antennas. These antennas can be utilised toform beams towards the ground thus illuminating “cells” that can beperceived by the user equipment as conventional terrestrial cells. Anadvantage of this approach relative to non-aerial vehicle basedcommunication networks is that the locations and density of cellscreated by the HAPs are dynamically controllable and do not involvechanges to the infrastructure on the ground.

A limitation on the spatial reuse of the wireless spectrum, and thus onthe capacity, of HAP based wireless networks is that the directionalityof the antennas mounted on HAPs are limited by their aperture size. Thephysical size and weight of an antenna mounted on a HAP is severelylimited by the payload requirements of any HAP in question.

The idea of overcoming the limitations imposed by use of a single HAPantenna beam width by using multiple HAPs illuminating the same area,and as a result boosting the broadband capacity in the area, has beenproposed by Grace et al “Improving the system capacity of broadbandservices using multiple high-altitude platforms” IEEE Transactions onWireless Communications, vol. 4, no. 3, pp. 700-709, March 2005. In thatproposed solution the directionality of user antenna is a key component.The user nodes are assumed to be static with highly directional antennase.g. dishes such that they are able to distinguish between differentHAPs in the sky. However, this approach does not achieve any capacitygains in a scenario where the user devices are not directional enough tobe able to point narrow beams to separate HAPs in the stratosphere abovethem. For example, this approach is not usable when HAPs are required toprovide connectivity to conventional cellular network user equipmenti.e. mobile phones with negligible directionality of their antennas.

GB2536015 suggests a manner for overcoming the spatial spectrum reuselimitation imposed by the size of antenna mounted on a HAP. GB2536015discusses the use of a so-called constellation of multiple HAPs toperform co-operative beamforming. In GB2536015 multiple HAPs spacedseveral kilometres apart adjust the phase of the signal to/from aparticular user equipment such that all copies of a signal arrive at areceiver in phase. This yields a power gain due to the coherent additionof multiple signals to/from every HAP. This works effectively as alarge, highly sparse phased array antenna. As a result, the perceived“effective antenna” formed by several HAPs span several kilometres. Thisallows the constellation of HAPs to create narrow cells at desiredlocations. However, in order to be able to create such narrow cells alocation of any user equipment where a beam is to be focused must firstbe accurately determined to a very high degree of accuracy. This initself is a complex problem which is made all the greater when the userequipment is mobile.

Conventionally a challenging aspect of forming such narrow inter-HAPcooperative beams has been the determination of the location of userequipment accurately enough. I.e. to within 50 cm or preferably within10 cm. Such beams which are formed cooperatively are an example of amulti-channel wireless communication link and can be considered asproviding a personal cell (P-cell). To be able to use such a small cellan approach is needed to associate a user with the P-cell and given thatthe cell is so small the cell must be able to track the location of theuser so as to avoid excessively frequent handovers between P-cells. If auser of user equipment is mobile both user acquisition and tracking isdifficult to achieve. A prior art approach of gradually narrowing downan existing beam by starting with few HAPs located close to one anotherand then adding more and more HAPs into a cooperative beamformingprocess (thus increasing the effective size of a sparse phased array ofHAP antennas) has been suggested. However such methodologies suffer fromvarious disadvantages such as the time taken to gradually narrow down anexisting beam.

It is an aim of the present invention to at least partly mitigate one ormore of the above-mentioned problems.

It is an aim of certain embodiments of the present invention to providea communication network and method of communicating whereby propagationcharacteristics for each respective wireless communication channelbetween a user equipment and aerial vehicles can be determined andthereafter used in a co-operative technique to connect user equipment toa core network via a multi-channel communication link.

It is an aim of certain embodiments of the present invention to providea process for estimating a wireless channel attenuation and phase shiftbetween multiple user equipment and multiple aerial vehicle antennas andusing this channel information to provide mobile coverage to the userequipment.

It is an aim of certain embodiments of the present invention to providemobile coverage to multiple user equipment in the same frequency-timespectrum.

It is an aim of certain embodiments of the present invention to increasea number of user equipment that can be supported by spatial multiplexingmethods over and above a fundamental limit otherwise imposed by aparticular number of aerial vehicles in use.

It is an aim of certain embodiments of the present invention to providean augmented methodology whereby interference effects, caused by furtherdevices which wirelessly communicate, but which are not part of acommunication network formed by a particular set of aerial vehicles anduser equipment, can be cancelled out thus improving a link quality foruser equipment services provided by the aerial vehicles.

It is an aim of certain embodiments of the present invention to providea method for connecting a mobile user equipment to a core network of atelecommunications network.

It is an aim of certain embodiments of the present invention to providea method for maintaining a connection of mobile user equipment to a corenetwork of a telecommunication network as the mobile user equipmentmoves.

It is an aim of certain embodiments of the present invention to providea communication network which is able to connect mobile user equipmentto a core network via a multi-channel communication link which providesa cell coverage area having a footprint with a width of less than 1 m.

It is an aim of certain embodiments of the present invention to providea communication network for maintaining connection of mobile userequipment to a core network as the mobile user equipment moves.

According to a first aspect of the present invention there is provided acommunication network, comprising:

-   -   a plurality of aerial vehicles that each supports at least one        respective directional antenna; and    -   a processing element for determining at least one propagation        characteristic for each respective wireless communication        channel between at least one user equipment and each aerial        vehicle of the plurality of aerial vehicles.

Aptly the processing element determines the propagation characteristic,for each wireless communication channel, via comparing an amplitude andphase of a received wireless signal transmitted via the respectivewireless communication channel with a predetermined reference amplitudeand reference phase.

Aptly the network further comprises a ground based cell processingcentre that comprises the processing element and includes at least oneinterface to a core network and optionally includes an aerial vehicleflight control unit, a channel estimation unit and a beamforming controlunit; and

-   -   at least one ground station, each comprising a directional        antenna element, arranged to relay user data and control        information between each aerial vehicle and the cell processing        centre.

Aptly the network comprises at least one user equipment.

Aptly the at least one propagation characteristic comprises an estimatedgain and an estimated phase shift of the wireless communication channel.

Aptly each user equipment is arranged to transmit a reference signalfrom the user equipment to each of the aerial vehicles.

Aptly the transmitted reference signal comprises a standardised networkreference signal.

Aptly the standardised network reference signal comprises an LTESounding Reference Signal.

Aptly each aerial vehicle is arranged to transmit a reference signalfrom the aerial vehicle to a user equipment.

Aptly each aerial vehicle comprises a high altitude platform (HAP).

Aptly the directional antenna of each aerial vehicle comprises at leastone multi element directional antenna array.

Aptly the processing element is located in a user equipment or an aerialvehicle.

Aptly the network further comprises a data store that stores a channelmatrix for wireless communication channels between every aerial vehicleand every user equipment, said channel matrix comprising a complexnumber for each wireless communication channel for each HAP-userequipment pair.

Aptly the data store stores a weight matrix for co-operative beamforming.

Aptly the network further comprises at least one multi-channel wirelesscommunication link each provided between each respective user equipmentand the plurality of aerial vehicles.

Aptly when a total number of user equipment exceeds a predeterminednumber, each user equipment is allocated a less than 100% share of abasic airtime bandwidth resource of a respective multi-channel wirelesscommunication link associated with that user equipment and the pluralityof aerial vehicles.

Aptly the network further comprises at least one further wireless devicethat is not included in a communication with the aerial vehicles via amulti-channel wireless communication link.

Aptly the processing element is arranged for determining at least onepropagation characteristic for each respective further wirelesscommunication channel between each further wireless device and eachaerial vehicle of the plurality of aerial vehicles.

Aptly the network further comprises a data store that stores a weightmatrix for beamforming when providing a multi-channel wirelesscommunication link between a user equipment and a plurality of aerialvehicles that includes weights determined responsive to the determinedpropagation characteristic of each further wireless communicationchannel.

Aptly each further device is a user equipment serviced via a cell otherthan a cell provided by the plurality of aerial vehicles or is a userequipment on a different access network to said a plurality of aerialvehicles.

According to a second aspect of the present invention there is provideda method of determining at least one propagation characteristic of eachwireless communication channel between at least one user equipment and aplurality of aerial vehicles, comprising the steps of:

-   -   for each respective wireless communication channel between each        of at least one user equipment and each aerial vehicle of a        plurality of aerial vehicles, determining an amplitude and phase        of a wireless signal transmitted between the user equipment and        an aerial vehicle associated with the respective wireless        communication channel; and    -   for each wireless communication channel, determining at least        one propagation characteristic of the wireless channel by        comparing the determined amplitude and phase with a        corresponding predetermined reference amplitude and reference        phase.

Aptly the method further comprises determining the at least onepropagation characteristic comprises determining an estimated gain andan estimated phase shift of the wireless communication channel.

Aptly the method further comprises for each wireless communicationchannel, transmitting a reference signal that comprises the wirelesssignal from the respective user equipment to the respective aerialvehicle and determining the amplitude and phase of the receivedreference signal responsive thereto.

Aptly the reference signal comprises a standardised network referencesignal.

Aptly the standardised network reference signal comprises an LTESounding Reference Signal.

Aptly the method further comprises, for each wireless communicationchannel, transmitting a reference signal that comprises the wirelesssignal from the aerial vehicle associated with the wirelesscommunication channel to the user equipment associated with the wirelesscommunication channel;

-   -   determining an amplitude and phase of the reference signal        received at the user equipment; and    -   transmitting a measurement report indicating the determined        amplitude and phase from the user equipment to the aerial        vehicle.

Aptly the method further comprises providing a channel matrix for all ofthe wireless communication channels between every aerial vehicle andevery user equipment that includes a complex number associated with eachrespective wireless communication channel for each aerial vehicle-userequipment pair.

Aptly each complex number has an amplitude that represents a channelamplitude gain for a respective wireless communication channel and anangle that represents a phase shift for that respective wirelesscommunication channel.

Aptly each aerial vehicle comprises a high altitude platform (HAP)located at least 5 km and optionally 15 km above sea level.

Aptly the method further comprises providing at least one wirelesscommunication link between each user equipment and the plurality ofaerial vehicles via a cooperative beamforming method responsive to saiddetermined propagation characteristics.

Aptly the method further comprises, when a total number of userequipment exceeds a predetermined number, allocating, to a userequipment, a less than 100% share of a basic airtime bandwidth resourceof the wireless communication link associated with that user equipment.

Aptly the method further comprises determining at least one propagationcharacteristic for each further wireless communication channel betweeneach of at least one further wireless device, that is not included incommunication with the aerial vehicles via wireless communication link,and each aerial vehicle of the plurality of aerial vehicles.

According to a third aspect of the present invention there is acomputer-readable storage medium comprising instructions which, whenexecuted by a computer, cause the computer to carry out a method ofdetermining at least one propagation characteristic of each wirelesscommunication channel between at least one user equipment and aplurality of aerial vehicles, comprising the steps of:

-   -   for each respective wireless communication channel between each        of at least one user equipment and each aerial vehicle of a        plurality of aerial vehicles, determining an amplitude and phase        of a wireless signal transmitted between the user equipment and        an aerial vehicle associated with the respective wireless        communication channel; and    -   for each wireless communication channel, determining at least        one propagation characteristic of the wireless channel by        comparing the determined amplitude and phase with a        corresponding predetermined reference amplitude and reference        phase.

According to a fourth aspect of the present invention there is provideda method of wireless communication between at least one user equipmentand a plurality of aerial vehicles, comprising the steps of:

-   -   for each respective wireless communication channel between at        least one user equipment and each aerial vehicle of a plurality        of aerial vehicles, determining an amplitude and phase of a        wireless signal transmitted between the user equipment and an        aerial vehicle associated with the respective wireless        communication channel;    -   for each wireless communication channel, determining at least        one propagation characteristic of the wireless communication        channel by comparing the determined amplitude and phase with a        corresponding predetermined reference amplitude and reference        phase;    -   for each wireless communication channel, determining, based on        the at least one propagation characteristic, at least one        weighting to be applied to signals transmitted and/or received        via a wireless communication link between the user equipment and        the aerial vehicles; and    -   providing a wireless communication link between the user        equipment and the plurality of aerial vehicles responsive to the        determined at least one weighting.

Aptly the method further comprises applying a respective weighting towireless signals transmitted from a transmitting aerial vehicle, via thefurther wireless communication link, thereby causing the transmittedsignals to arrive at the user equipment substantially in phase withcorresponding wireless signals arriving at the user equipment from atleast one further aerial vehicle.

Aptly applying the respective weighting further causes the transmittedsignals to arrive at the user equipment substantially out of phase withwireless signals arriving at the user equipment from at least oneinterference source.

Aptly the method further comprises applying a corresponding weighting toreceived signals received at the aerial vehicle via the wirelesscommunication link thereby causing the received signals to besubstantially in phase with signals received from the user equipment atat least one further aerial vehicle.

Aptly applying the corresponding weighting further causes the receivedsignals to be substantially out of phase with interference wirelesssignals received at the aerial vehicle from at least one interferencesource.

Aptly at least one propagation characteristic for each wirelesscommunication channel comprises an estimated gain and a phase shiftassociated with that wireless communication channel.

Aptly the method further comprises providing the wireless communicationlink via a cooperative beamforming method performed by the plurality ofaerial vehicles.

Aptly beamforming comprises applying a minimum mean square errortechnique.

Aptly beamforming comprises applying a zero forcing technique.

Aptly the method further comprises, when a total number of userequipment exceeds a predetermined number, allocating, to a userequipment a less than 100% share of a basic airtime bandwidth resourceof the wireless communication link associated with that user equipment.

Aptly the method further comprises determining at least one propagationcharacteristic for each further wireless communication channel betweeneach of at least one further wireless device, that is not included incommunication with the aerial vehicles via a wireless communicationlink, and each aerial vehicle of the plurality of aerial vehicles.

According to a fifth aspect of the present invention there is provided acomputer readable storage medium comprising instructions which, whenexecuted by a computer, cause the computer to carry out a method ofwireless communication between at least one user equipment and aplurality of aerial vehicles, comprising the steps of:

-   -   for each respective wireless communication channel between at        least one user equipment and each aerial vehicle of a plurality        of aerial vehicles, determining an amplitude and phase of a        wireless signal transmitted between the user equipment and an        aerial vehicle associated with the respective wireless        communication channel;    -   for each wireless communication channel, determining at least        one propagation characteristic of the wireless communication        channel by comparing the determined amplitude and phase with a        corresponding predetermined reference amplitude and reference        phase;    -   for each wireless communication channel, determining, based on        the at least one propagation characteristic, at least one        weighting to be applied to signals transmitted and/or received        via a wireless communication link between the user equipment and        the aerial vehicles; and    -   providing a wireless communication link between the user        equipment and the plurality of aerial vehicles responsive to the        determined at least one weighting.

Certain embodiments of the present invention provide a communicationnetwork that includes a processing element, which may be in an aerialvehicle or in a ground based processing centre, that can determine oneor more propagation characteristics for wireless communication channelsassociated with respective multi-channel and/or single channel wirelesscommunication links between ground based user equipment and aerialvehicles. This data can be used to create a channel matrix and a pseudoinverse channel matrix which can be selectively applied to transmittedand received signals in the uplink and downlink direction allowinginterference free concurrent data communication with all user equipment.

Certain embodiments of the present invention provide a method ofdetermining at least one propagation characteristic for each wirelesscommunication channel between one or more user equipment and a pluralityof aerial vehicles.

Certain embodiments of the present invention provide a communicationnetwork which includes multiple aerial vehicles, such as a constellationof HAPs, a processing element in an aerial vehicle or in a processingcentre, and optionally one or more ground stations that can be used asrelays to relay user data and control information between every HAP andthe processing element.

Certain embodiments of the present invention provide for a groupingscheme whereby a number of user equipment that can be supported forwireless communication is greater than that which might otherwise besupported by the specific number of aerial vehicles provided. Aptly thisgrouping scheme comprises a user time-frequency resource schedulingmethodology at times when a number of user equipment exceeds apredetermined maximum number of possible concurrent data transmissions.

Certain embodiments of the present invention enable interference from/toother wireless communication devices, that do not form part of thecommunication network served by a plurality of aerial vehicles, to becancelled out as part of the process by which the aerial vehiclescommunicate with their designated user equipment. This helps improvecommunication quality for all devices.

Certain embodiments of the present invention provide a method andcommunication network for connecting mobile user equipment to a corenetwork whereby communication occurs initially via a single channelwireless communication link, which provides a cell coverage area whichis relatively large, and thereafter, via a handover step, communicationoccurs via a multi-channel wireless communication link with a smallercell coverage area.

Certain embodiments of the present invention provide a method andcommunication network for maintaining a connection of a mobile userequipment to a core network as the user equipment moves. Communicationoccurs via a multi-channel communication link with a relatively smallcell coverage area and then when it is determined that the userequipment is moving in a way which means loss of connectivity is likely,a handover step occurs transferring communication to communication via asingle channel communication link which has a larger cell coverage area.Thereafter, optionally, a further handover step can occur to transfercommunication back to a multi-channel communication link which has asmall cell coverage area if/when movement is such that a small cellcoverage area is likely to be able to maintain a link.

Certain embodiments of the present invention provide a communicationnetwork which comprises at least one airborne antenna for transmittingand receiving signals between the antenna and a user equipment and atleast one antenna for transmitting and receiving signals with a basestation which is connection with a surface mounted processing centre.The processing centre includes a control interface and a cellularinterface in connection with a core network.

Certain embodiments of the present invention enable a user trackingmethodology for maintaining a connection of mobile user equipment to acore network.

Certain embodiments of the present invention provide a process foracquiring a mobile network user via a wide coverage cell, determiningtheir location using aerial antennas mounted on HAPs and handing overthe user device to a narrow cell formed by aerial antennas mounted ontwo or more HAPs.

Certain embodiments of the present invention provide apparatusperforming precise channel estimation exploiting some conventionalcellular signalling and aerial antennas mounted on multiple HAPs toallow user locations to be detected to an accuracy of less than 0.5 m(that is to say considerably more accurately than conventionaltechniques allow).

Certain embodiments of the present invention provide for the use ofmultiple HAPs that transmit overlapping beams containing beacons withdifferent physical cell IDs (or synchronised sequences).

Certain embodiments of the present invention provide for the use ofbeacon signals from different HAPs for user equipment-assistedpositioning. A user equipment can be requested to measure and observetime difference of arrival (OTDoA) of signals from different HAPs andreport that back to a serving cell base station.

Certain embodiments of the present invention provide for a process ofusing receiver antennas mounted on multiple HAPs to estimate a userlocation based on an uplink reference signal (URS).

Certain embodiments of the present invention enable the formation andsteering of ultra-narrow cells to within 0.5 m of a user's location.

Certain embodiments of the present invention provide for the handover ofa user from a wide coverage cell to an ultra-narrow cell formed byaerial antennas mounted on multiple HAPs.

Certain embodiments of the present invention provide for a handover of auser equipment from an ultra-narrow cell, formed by aerial antennasmounted on multiple HAPs, to a wide coverage cell formed by fewer HAPsor optionally by a single HAP.

Certain embodiments of the present invention provide a process andapparatus for enabling cellular devices to be simultaneously connectedwith a first cell formed by aerial antennas mounted on multiple HAPs andto a further cell formed by fewer HAPs (and optionally a single HAP) ora terrestrial base station.

A user device location can be defined either geographically or in termsof a phase shift and attenuation of Radio Frequency (RF) signals betweenthe user device and aerial antennas mounted on HAPs. Thus, for certainembodiments of the present invention, if a user device does not haveline-of-sight connection with the HAPs but uses reflected and/orshadowed radio propagation paths, a user's apparent location cannevertheless be determined and expressed as a channel phase shift andattenuation of the reflected/shadowed paths between the user and theHAPs. This information can be used to form a narrow cell using aerialantennas mounted on two or more HAPs.

Certain embodiments of the present invention will now be describedhereinafter, by way of example only, with reference to the accompanyingdrawings in which:

FIG. 1 illustrates multiple aerial vehicles providing cellular coverageto user equipment via multiple possible single channel wirelesscommunication links;

FIG. 2 illustrates a constellation of HAPs providing two multi-channelwireless communication links to respective user equipment;

FIG. 3 schematically illustrates a way for estimating a wirelesschannel;

FIG. 4 schematically illustrates an alternative way for estimating awireless channel;

FIG. 5 illustrates downlink channel estimation after a P-cell, providedby a multi-channel wireless communication link, has been established andusing measured arrival time of downlink signals;

FIG. 6 illustrates channel estimation after a P-cell, provided by amulti-channel wireless communication link, has been established andusing uplink reference signals;

FIG. 7 illustrates a channel matrix;

FIG. 8 illustrates a weight matrix;

FIG. 9 illustrates grouping of user equipment;

FIG. 10 illustrates interference cancelling;

FIG. 11 illustrates a HAP-Cell system integrated with a cellulararchitecture and illustrates certain selected elements associated with acore network node in more detail;

FIG. 12 illustrates user acquisition using downlink channel estimation;

FIG. 13 illustrates user acquisition using uplink channel estimation;

FIG. 14 illustrates user acquisition apparatus;

FIG. 15 illustrates user equipment tracking using downlink controlsignals;

FIG. 16 illustrates user tracking apparatus;

FIG. 17 illustrates a handover step used to avoid loss of connectivity;

FIG. 18 illustrates user equipment location tracking apparatus;

FIG. 19 illustrates a user equipment simultaneously communicating withboth a wide coverage cell and a narrow P-cell to help maintain a robustconnection and avoid loss of connectivity; and

FIG. 20 illustrates user acquisition and channel estimation without aninitial aerial vehicle link to user equipment.

In the drawings like reference numerals refer to like parts.

Certain embodiments of the present invention provide for high speedbroadband services from high altitude platforms (HAPs). A HAP may be anaircraft or lighter than air structure 10 to 35 km above sea level. AHigh Altitude Long Endurance (HALE) aircraft or free flying or tetheredaerostat can be an example of a HAP. A HAP is an example of an aerialvehicle. Other such aerial vehicles such as tethered vehicles or mannedaircraft or the like can be utilised according to other embodiments ofthe present invention. Aptly each aerial vehicle is deployed at least 5km above sea level. Aptly each aerial vehicle can be deployed in thestratosphere at an altitude above sea level of around 17 to 22 km. HAPscover significantly wider areas with Line-of-Sight (LoS) links comparedto conventional terrestrial systems and do not suffer from capacity andpropagation delay limitations associated with satellites.

FIG. 1 illustrates how multiple aerial vehicles 100 ₁₋₃ (three HAPsshown in FIG. 1 ) can provide wireless services to user equipment. Auser equipment can be a mobile phone, tablet or laptop or PDA or thelike. Each user equipment described hereinbelow, by way of example only,is a smartphone. Each aerial vehicle described hereinbelow, by way ofexample only, is a HAP. Each HAP shown is equipped with one or moredirectional antenna 105,110. Aptly each directional antenna is amulti-element phased array antenna. Such antenna and their generalcontrol is described in GB2536015 the disclosure of which isincorporated herein by reference.

As shown in FIG. 1 each aerial vehicle 100 alone can form one or morebeams 120 which are directed to the ground thus illuminating ‘cells’.The beams provided by a single HAP can be used to provide a singlechannel wireless communication link between the respective HAP and auser equipment in each cell. The wireless communication link is two wayor bidirectional in the sense that uplink and downlink transfer of datacan be supported. The cells 130 can be perceived by the user equipmentas conventional terrestrial wireless telecommunication cells. Thelocations and density of cells created by the HAPs are dynamicallycontrollable. Each aerial vehicle supports at least one directionalantenna. In FIG. 1 each vehicle supports a transmission antenna 105 anda reception antenna 110. Optionally there can be phased array antenna orhorns. Each phased array antenna can comprise an array of small antennaelements.

Each single channel wireless communication link is formed over thechannel between a single user equipment and a single HAP. The singlechannel communication link is provided by forming a beam, from adirectional antenna of a respective aerial vehicle, towards the groundthat illuminates a first cell coverage area. The first cell coveragearea has a relatively wide footprint and may thus be referred to as awide cell. Aptly the footprint has a width of greater than 500 m. Thesingle channel communication link enables synchronisation and/orassociation and/or exchange of control signals between a respective userequipment and a respective HAP and a core network.

A footprint of a first cell coverage area provided by any one HAPcomprises a region where a wireless signal strength is strong enoughthat a user equipment located within the cell coverage area canassociate with the core network via an associated wireless communicationlink. Aptly a footprint of a cell coverage area is a region defined byan imaginary boundary congruent with positions where a wireless signalstrength of a communication link is at a predetermined threshold levellower than a maximum signal strength in the cell coverage area.Optionally the predetermined threshold is around 9 dB.

Optionally the HAPs 100 utilise co-operative aerial inter-platformbeamforming (CAIB). Such a co-operative beamforming scheme can beperformed across multiple HAPs to decrease beam size and increase radioresource usage efficiency of the radio spectrum. With this technique itis possible for multiple aerial vehicles to create effective beams thatprovide a wireless communication link between those multiple HAPs and asingle user equipment. These co-operatively formed effective beams canbe provided for many individual users illuminated via smaller cells.These smaller cells have a footprint that can be as small as 10 to 100cm in width. As such the cells may be thought of as personal cells(P-Cells) as their scale approximates to the size of a person using theuser equipment. The same group of HAPs can simultaneously providemultiple cooperatively formed beams for multiple smartphones. Thisprovides multiple respective multi-channel wireless communication links.

Each multi-channel wireless communication link is formed across themultiple channels between any one user equipment and the HAPs in aconstellation of HAPs which cooperatively beam form to create themulti-channel wireless communication link. The multi-channelcommunication link enables synchronisation and/or association and/orexchange of control signals. Each multi-channel communication link canbe provided by cooperatively forming a beam, via respective directionalantennas of a plurality of aerial vehicles, towards the ground thatilluminates a cell coverage area.

A single channel communication link provides a first cell coverage area.A multi-channel communication link illuminates a further cell coveragearea. A further cell coverage area has a footprint which is much smallerthan the associated footprint provided by a single channel communicationlink. Aptly the further cell coverage area which is a cell coverage areaprovided by a multi-channel communication link has a footprint with awidth of less than 1 m. Optionally the further cell coverage area has afootprint with a width of less than 0.5 m. These may be thought of asnarrow or ultra-narrow cells.

A footprint of the further cell coverage area provided by amulti-channel communication link comprises a region where a wirelesssignal strength is strong enough that a user equipment located in thefurther cell coverage area can associate with a core network via anassociated multi-channel communication link. Aptly a footprint of afurther cell coverage area comprises a region defined by an imaginaryboundary congruent with positions where a wireless signal strength ofthe multi-channel wireless communication link is at a predeterminedthreshold level lower than a maximum signal strength in the further cellcoverage area. Aptly the predetermined threshold is around 9 dB.

Each wireless communication link is thus provided via one or morerespective channels. A channel is a pathway or medium through which datacan be wirelessly communicated between any two points. Beams providedbetween a single HAP and a single user equipment are single channel inthe sense that they have a single source and a single sink. Beamsprovided between a single user equipment and multiple HAPs aremulti-channel in the sense that they have multiple sources and a singlesink or a single source and multiple sinks (depending upon whether auplink or downlink transmission is occurring).

FIG. 2 schematically illustrates a plurality (referred to as aconstellation 200) of HAPs (four shown in FIG. 2 ) together providingtwo simultaneous multi-channel wireless communication links 210. Eachmulti-channel wireless communication link 210 is provided via aco-operative beamforming technique with a respective user equipment220,230 as its target. For the example shown in FIG. 2 this provides tworespective P-Cells 240,250 each formed on a location of a respectiveuser equipment. Each P-Cell 240, 250 has a footprint that is muchsmaller in area than a corresponding footprint of a cell 130 provided bya single HAP alone via a respective single channel wirelesscommunication link provided without beamforming with other HAPs.

As illustrated in FIG. 2 the constellation 200 of HAPs communicatewirelessly with one or more ground station 255 (one shown in FIG. 2 ).Each ground station 255 includes a directional antenna 260 and theground station can relay, using a respective wireless connection 262,user data and control information between every HAP and a cellprocessing centre 265. The processing centre 265 shown in FIG. 2 is aHAP-Cell processing centre. This may be located adjacent to or be partof the ground station or, as shown in FIG. 2 , may be connected to theground station/s 255 via a wired connection 270. Optionally a wirelessconnection can connect the ground station/s to the processing centre.

The HAP-Cell processing centre is provided with significantcomputational capability. The processing centre controls the operationof the HAP-based access network. The processing centre 265 can include abeamforming control unit 275 that runs processes for beamformingcontrol. The processing centre can include a HAP flight control unit 280that runs processes for controlling the flight of each aerial vehicle(such as location flight path and/or altitude). The processing centre265 includes at least one interface 285 with a core network 290 via arespective wired or wireless connection 292.

For providing co-operative beamforming across aerial antennas mounted onthe HAPs accurate channel estimation for every wireless communicationchannel between each HAP and each user equipment is performed. This isprocessed via a processing element 295 in the processing centre 265.This can be achieved in different ways as described below using wirelesssignals on either the downlink or the uplink. The processing element 295may be a processor. This is used to indicate a central processing unit(CPU) or electronic circuit within a computer or a computer configuredto carry out instructions of a computer programme. It will be understoodin what follows that stating that a processor implements an action orthat the processor is configured to implement an action is analogous tostating that the processor is configured to execute computer readableinstructions and implement or execute an action. It is likewise to beunderstood that the term “computer” is intended to mean an assembly ofcomponents e.g. a processor, memory element, input device and outputdevice arranged to receive inputs, produce outputs, store informationand algorithms and execute instructions. It is not intended to belimited to any specific arrangement or group of components. For theavoidance of doubt the processor may optionally be a general purposeprocessor, co-processor or special-purpose processor such as, forexample, a network or communication processor, compression engine, highthroughput many integrated core co-processor, embedded processor or thelike. The processor may be implanted on one or more chips. The chips canbe proximate to one another or interconnected at different locations.The processor may be a part of and/or may be implemented on one or moresubstrates using any number of processed technologies, such as, forexample, BiCMOS, CMOS or EMOS.

FIG. 2 thus helps illustrate a communication network 296. Thecommunication network includes a plurality of aerial vehicles (HAPsshown in FIG. 2 ) that each supports at least one respective directionalantenna. A processing element 297 (shown in FIG. 2 as part of thechannel estimation unit 295) for determining at least one propagationcharacteristic for each respective wireless communication channelbetween at least one user equipment and each aerial vehicle of theplurality of vehicles. The processing element 297 determines thepropagation characteristic, for each wireless communication channel.This can be achieved by comparing an amplitude and phase (equivalent tocomparing an indicator of amplitude and/or an indicator of phase) of areceived wireless signal transmitted via the respective wirelesscommunication channel with a predetermined reference amplitude andreference phase or associated indicator. The communication networkfurther includes a ground based cell processing centre 265 (that in FIG.2 includes the processing element) and includes at least one interfaceto a core network 290. Optionally the ground based cell processingcentre 265 includes an aerial vehicle flight control unit 280 and abeamforming control unit 275. It will be appreciated that flight controland beamforming control and channel estimation could be carried out atan alternative node or other alternative nodes in the overallcommunication network. The communication network 296 also includes atleast one ground station 255. Each ground station includes a directionalantenna element 260. The ground station is arranged to relay user dataand control information between each aerial vehicle and the cellprocessing centre 265. As illustrated in FIG. 2 the communicationnetwork may also comprise at least one user equipment in the form of asmartphone, mobile phone, tablet or the like. FIG. 2 illustrates twosmartphones.

FIG. 3 schematically illustrates a way for estimating the wirelesschannels between a user equipment and multiple HAPs. As illustrated inFIG. 3 each HAP (five shown in FIG. 3 ) 100 wirelessly communicates witha user equipment via a respective wireless communication channel 300₁₋₅. Each HAP transmits a predetermined reference signal as a beaconsignal (shown schematically via a common line 310) which is individuallydetectable by the user equipment in question. The user equipmentmeasures the received signal strength and relative time difference ofarrival of each reference signal. Optionally the user equipment measuresa time of arrival for each reference signal. Optionally the userequipment relays received packets of each reference signal. Aptly thereference signal comprises a reference sequence. The user equipmentsends back a respective measurement report via an existing communicationlink (provided in a way described in more detail below) indicatingpropagation characteristics for every separate reference signal itreceives from the HAPs. The measurement report thus includes informationwhich can be used to generate an estimated channel matrix. A channelmatrix can be constructed representing characteristics for all thewireless communication channels between every HAP and every userequipment. Optionally each HAP-user equipment pair has a complex numberassociated with it. The amplitude of the complex number represents therespective channel amplitude gain (square root of the power gain). Theangle in the complex plane represents the phase shift of the channel. Itwill be appreciated that whilst the channel matrix can be represented bymultiple complex numbers other mathematical techniques can be utilisedto store indicators associated with channel amplitude gain and phaseshift associated with any single channel for the multiple channelsbetween user equipment and aerial vehicles in a communication network.Via the estimation scheme illustrated in FIG. 3 a user equipment onlyneeds send a single report to any one HAP with a whole list of other HAPreference signal times. The user equipment does not need tocommunication with every HAP separately. A user equipment is registeredand communicating via a single cell be it a wide coverage cell or aP-cell. It will be appreciated that a possible alternative isco-operative multi-point where a single user might be served by severalcells at once.

FIG. 4 schematically illustrates an alternative way for estimating thewireless channel between a user equipment and multiple HAPs. Asillustrated in FIG. 4 each HAP (five shown in FIG. 4 ) 100 wirelesslycommunicates via a respective communication channel and a user equipmentis instructed, either via a single channel wireless communication linkfrom selected HAP, or subsequent to establishing a multi-channelwireless communication link via co-operative beamforming via that link,to transmit a predetermined reference signal. Aptly the reference signalis a sounding reference signal as standardised in Long Term Evolution(LTE) networks. This reference signal is then received wirelessly viarespective channels by every individual HAP. The received data can berelayed or data extracted from the received signal is relayed to aprocessing element in the HAP-Cell processing centre which can thenestimate the uplink channel attenuation and phase shift. An advantage ofthis method over the downlink estimation method described with respectto FIG. 3 is that its accuracy depends upon the implementation of theHAP-Cell system and is not limited by the measurement accuracy ofstandard mobile user equipment. Aptly content associated with thereceived reference signal is not extracted. Only relative amplitude andphase associated with a transmitted data packet forming part of areference signal is utilised. A whole packet may be relayed to a groundstation in a synchronised way or amplitude and time of arrival of areceived signal can be measured at each HAP and the measured values arerelayed. Time reference across different HAPs can be synchronised tohelp provide accurate signalling. Synchronisation can be achieved viaone or more of reference to GPS time reference, atomic clocks,synchronisation signals from ground stations (which are aware ofpropagation delay differences to every HAP) or the like.

The processing element of the communication network determines at leastone propagation characteristic for each respective wirelesscommunication channel. Aptly the at least one propagation characteristiccomprises an estimated gain and an estimated phase shift of the wirelesscommunication channel. Optionally each user equipment is arranged totransmit a reference signal from the user equipment to each of theaerial vehicles. The transmitted reference signal may comprise astandardised network reference signal. Aptly the standardised networkreference signal can comprise an LTE sounding reference signal.Alternatively each aerial vehicle can be arranged to transmit areference signal from the aerial vehicle to a user equipment.

FIG. 5 helps illustrate downlink channel estimation after a P-cell hasearlier been established. This corresponds to the downlink channelestimation scheme illustrated in FIG. 3 . FIG. 5 illustrates two nodesin a communication network and the signaling and actions therebetween. Afirst node represents the user equipment node 500 and the further node510 represents the HAP-Cell system which schematically is associatedwith a constellation of HAPs 200, the ground station 255 and HAP-Cellprocessing centre 265 shown in FIG. 2 . Via a first step S 515 theHAP-Cell processing centre instructs the user equipment to measure timedifference of arrival (TDoA) of beacon signals sent from a plurality ofHAPs. Each of those HAPs is associated with the narrow cell oralternatively may not be contributing to that cell. At step S 520 theHAP-Cell system including the HAPs transmit beacon signals in thevicinity of the user equipment. The user equipment determines amplitudeand time difference of arrival of any detected beacon signals from thevarious HAPs and from this sends back a measurement report via step S530 the measurement report can optionally include amplitude/phaseinformation determined at the user equipment itself or may optionallyinclude information in the form of raw data that can subsequently beused at the node 510 associated with the HAP-Cell system. Responsive tothe measurement report at step S 540 the downlink channelcharacteristics can be estimated with the estimated values being used tohelp create a channel matrix. An uplink channel can be estimatedresponsive to the estimated downlink channel. This is illustrated bystep S 545. The uplink channel can be estimated by assuming that thesignal attenuation and phase shift between user equipment and HAP isidentical on both uplink and downlink. The channel estimation steps arerepeatedly estimated via an iteration step S 550 which may occur atrepeated intervals of between ever 100 milliseconds to every 1 second.

FIG. 6 illustrates channel estimation after a P-cell has earlier beenestablished using uplink reference signals. FIG. 6 illustrates theexchange of signals between a node 500 in the communication networkassociated with user equipment and a node 510 in the communicationnetwork associated with a HAP-Cell system. This latter node 510 includesthe aerial vehicles and ground station and HAP-Cell processing centre265 illustrated in FIG. 2 .

At step S 615 the HAP-Cell system is shown to instruct a user equipmentto transmit uplink reference signals (URS) at regular intervals. Theseinstructions are duly received at the user equipment node 500 which thenbegins to transmit URS at step S 625. At step S 630 the HAP-Cell systemestimates uplink channel characteristics based on amplitude and phasedifference of URS received at multiple HAPs. Thereafter, illustrated viastep S 640, the downlink channel can be estimated based on an uplinkchannel estimate. The downlink channel is estimated by assuming that thesignal attenuation and phase shift between the user equipment and HAP isidentical on both uplink and downlink. FIG. 6 helps illustrates how thetransmission of URS and uplink and downlink channel estimation isrepeated iteratively at intervals of between 10 milliseconds and 1second. Because a burden of processing the reference signal is at theHAP-Cell system side rather than in a conventional user equipment(smartphone etc.) iterations can be more frequent. Intervals of 1 ms ormore can be used. This iteration is illustrated in FIG. 6 via step S650.

A purpose of the channel estimation process is to construct a channelmatrix at least for the wireless communication channels between everyHAP and every user equipment. A structure for a channel matrix 700 isshown in FIG. 7 . Every HAP-user equipment pair has a channelcoefficient 710 associated with it. This coefficient is a complex numberwhose amplitude represents the respective channel gain and whose phaserepresents the respective channel phase shift. In this way, the receivedsignal at every user equipment can be calculated using the followingequation:y=Hx   (1)where H is a channel matrix, x is a column vector of transmitted signalsfrom every HAP, and y is a column vector of received signals at everyUE. The received signal power for every user equipment from the giventransmit signal vector can be calculated by taking the square of theamplitude of the received signal vector as follows:P _(Rx) =|y| ²  (2)

In the case of the downlink, the task of the function is to use theestimated version of the channel matrix H to compose the transmit vectorx, such that the wanted signals are successfully reconstructed at theirrespective user equipment receiver locations. A matrix W can then bederived, of the form shown in FIG. 8 , to transform the signals requiredto be transmitted to every user equipment into the transmit signal forevery HAP such that the original signals are then reconstructed at theuser equipment receivers.

The composition of the transmit signal vector using the matrix can beexpressed as follows:x=W√Qs   (3)where W is the matrix 800 of the form shown in FIG. 8 , Q is a diagonalpower allocation matrix, and s is a column vector of transmit symbolsfor every user equipment. In this way every column in the W√Q matrixindicates the amplitude and phase shift of the signal at every HAPrequired to communicate with a particular single user equipmentreceiver.

In the case of uplink (equalisation), x is the transmit vector ofsignals from all user equipment and y is the vector of received signalsat every HAP. Consequently the dimensions of the channel matrix Hand theweight matrix Ware inverted for mathematical compatibility. The job ofthe equalisation algorithm is then to recover the transmitted signalsper user given the received signals at every HAP as follows:x _(recovered) =Wy   (4)

If the channel matrix H can be estimated by the HAP-Cell system, it canbe used to derive the weights for different approaches. For example, anapproach that can be used and that aims to maximise the received poweris Maximum Ratio Combining (MRC). Strictly MRC refers to combiningsignals from multiple HAPs on the uplink: the equivalent applied to thedownlink can be referred to as Maximum Ratio Transmission (MRT).However, in this description the term MRC is used for both:W _(MRC) =kH ^(H)  (5)where H^(H) is the Hermitian (i.e. conjugate transpose) of the channelmatrix H, and k is a single normalisation factor for the entire matrix.

However, adopting the complex channel matrix approach described meansother methods with interference cancellation properties, can be used.For example, a weight matrix for a zero forcing approach is given below:W _(ZF) =kH ^(H)(HH ^(H))⁻¹  (6)

In this way, the weight matrix W_(ZF) is a scaled pseudo-inverse of thechannel matrix H. Given, perfect channel estimation and a smaller numberof user equipment than HAPs, this approach can achieve perfect or nearperfect interference cancellation, i.e. the signal arriving at every UEis free of interference from concurrent signals to every other userequipment.

An alternative to using zero forcing and that achieves a good balancebetween maximising the received power and cancelling interference fromother transmissions is minimum mean square error (MMSE). The weightderivation formula for MMSE is given below:W _(MMSE) =kH ^(H)(HH ^(H)+σ² I)⁻¹  (7)where I is the identity matrix and a² is the noise variance termcalculated as follows:

$\begin{matrix}{\sigma^{2} = \frac{P_{Noise}}{P_{Tx}}} & (8)\end{matrix}$where, P_(Tx) is transmit power (necessary for normalisation in thedownlink precoding case to optimise SNR at the remote receivers, thisfactor is not applicable in the uplink equalisation case because therethe HAPs are the receivers); and P_(Noise) is the noise power at thereceiver (this includes external interference from other patches/cells,or in general from all other transmissions not included in the given Hmatrix).

This noise enhancement term helps provide adaptable trade-off betweeninterference cancellation and received signal power. In high SNRscenarios, it is negligible and MMSE behaves as zero forcing; however,in low SNR scenarios, this noise variance term makes MMSE operate in apower maximizing mode.

The channel estimation unit 295 in the ground based processing centre265 includes a processing element 299 for determining at least onepropagation characteristic for each respective wireless communicationchannel. A data store 298 in the channel estimation unit 295 stores achannel matrix for wireless communication channels between every aerialvehicle and every user equipment. The channel matrix comprises a complexnumber for each wireless communication channel for each aerialvehicle-user equipment pair.

Estimating the wireless communication channel between every HAP andevery user equipment enables further beamforming methods withinterference cancellation to be deployed among user equipment in closeproximity to one another. FIG. 7 helps illustrate a structure of abeamforming channel matrix that can be utilised. In the downlink case, acomplex matrix 700 such as this can be used to transform transmitsignals to every user equipment into the actual transmit signals issuedfrom every HAP. In this way wanted signals are reconstructed at everyindividual user equipment and unwanted interference is cancelled out. Inthe uplink case a complex weight matrix 800 (as shown in FIG. 8 ) can beused to reconstruct signals received from every user equipment given themixture of the signals received at every HAP. In this way taking theinverse or pseudo inverse of the channel matrix such as that depicted inFIG. 7 and using the result as a beamforming weight matrix may bereferred to as zero forcing. If a number of user equipment is lower thana number of HAPs and if the channel is estimated accurately then it istheoretically possible for all interference to be cancelled out. Thusinterference free concurrent data communication can be allowed with alluser equipment. I.e. spatial multiplexing from HAPs.

Certain embodiments of the present invention thus enable beamformingmethods based on complex channel estimation to achieve network capacityenhancements relative to prior art techniques by enabling high data rateconcurrent communication with multiple user equipment. A number of userequipment that can be supported by these spatial multiplexing methodscan be limited by a number of HAPs in a serving constellation. Forexample if there are 50 HAPs used for co-operative interplatformbeamforming, a number of user equipment that can be independentlyconcurrently supported with interference cancellation is theoreticallyless than 50.

FIG. 9 helps illustrate a technique for grouping user equipment intoscheduling clusters to help overcome this limitation. Thus, certainembodiments of the present invention overcome this limitation andmaintain network throughput enhancement achieved by the HAP basedspatial multiplexing method by using a user time-frequency resourcescheduling methodology. This methodology is optionally applied where anumber of user equipment exceeds a predetermined or optimal number ofconcurrent data transmissions. FIG. 9 illustrates an example where thereare four user equipment 910, 920, 930, 940 requiring data communicationbut in a hypothetical scenario when an optimal number ofinterference-free concurrent transmissions is three. In this instancethe user equipment can be grouped by adopting airtime schedulingclusters. An optimal number of concurrent transmissions can be definedas a maximum number of users in a beamforming cluster, for whicheffective interference cancellation is achievable. This is such that thetotal throughput of all concurrent transmissions is at its maximum. Thisnumber will vary, and will typically be less than the number of HAPsinvolved in beamforming. It will be adjusted dynamically by the HAP-Cellprocessing centre based on the number and position of the HAPs, thesignal-to-noise ration and the measured channel characteristics of everyuser equipment.

For example, 25% of airtime can be allocated to a first schedulingcluster for users of three user equipment 910, 920 and 930. 25% ofairtime can be allocated to a second scheduling cluster for users ofanother three user equipment of 920, 930 and 940. 25% of airtime can beallocated to a third scheduling cluster for users of a still furtherthree user equipment 910, 930 and 940. 25% of airtime can be allocatedto a fourth scheduling cluster for users of a remaining three userequipment 910, 920 and 940.

In this way every user equipment is allocated a total of 75% of thebandwidth resources with three out of four users always communicatingsimultaneously using the HAP based beamforming methods. It will beappreciated that this grouping methodology can be scaled up where everyuser will optimally be allocated a proportion of airtime resource:

$\begin{matrix}{{{Proportion}\mspace{14mu}{of}\mspace{14mu}{airtime}} = \frac{{Optimal}\mspace{14mu}{number}\mspace{14mu}{of}\mspace{14mu}{concurrent}\mspace{14mu}{transmissions}}{{Total}{\mspace{11mu}\;}{number}\mspace{14mu}{of}\mspace{14mu}{users}}} & (9)\end{matrix}$

FIG. 10 illustrates an alternative to the clustering methodologyillustrated in FIG. 9 . In contrast with the scenario described withrespect to FIG. 9 where the number of simultaneously served usersexceeded a spatial multiplexing capacity of a HAP-Cell system it will beappreciated that there are situations where a number of user equipmentis lower than a predetermined number of concurrent transmissions. Forexample, this may be the case in sparsely populated rural areas. Forthese scenarios certain other embodiments of the present invention makeuse of the inclusion of other radio transceivers/transmitters in thechannel estimation and beamforming methods previously described. Thesefurther receivers/transmitters are accounted for the sole purpose ofinterference cancellation. That is to say data is not communicated withthese further devices.

FIG. 10 helps depict an example of this approach. As shown only twousers each with a respective user equipment 1010, 1020 communicate withthe constellation of HAPs. In this hypothetical scenario this is below aspatial multiplexing capacity of the HAP-Cell system. As a result acorresponding beamforming weight matrix can be augmented to includeentries for one or more further wireless devices. One further wirelessdevice 1030 is illustrated in FIG. 10 . This further wireless device 830may be representative of a user with a user equipment in a differentcell or on a different access network operating in the same spectrum. Inthis way the interference from/to the further wireless device will becancelled out by the beamforming process. This helps improve quality ofa wireless communication link between the constellation of HAPs and theuser equipment 1010, 1020 as well as helping improve communicationquality at the further additional device 1030.

Aptly when a total number of user equipment exceeds a predeterminednumber associated with a number of deployed HAPs, each user equipment isallocated a less than 100% share of a basic airtime bandwidth resourceof a respective wireless communication link associated with that userequipment and the plurality of aerial vehicles. This technique forsharing a total basic airtime bandwidth resource by grouping userequipment into scheduling clusters helps a predetermined number of HAPsprovide wireless communication to a relatively large number of userequipment. Aptly the processing element of the channel estimation unitis arranged for selectively determining at least one propagationcharacteristic for each respective further wireless communicationchannel between further wireless devices and each aerial vehicle of aplurality of aerial vehicles served by the processing centre 265. Thatis to say when the processing centre handles communication with userequipment via a constellation of aerial vehicles further wirelessdevices which may be wireless devices operating on another network havepropagation characteristics determined for those further wirelesscommunication channels. A signal transmitted by a third party device canbe detected (but not decoded) at individual HAPs and time of arrival andsignal strength can be compared. This is equivalent to uplink channelestimation. As an alternative approximating a channel based on ageographical location can be utilised. This will be a best effortinterference cancellation technique. A still further alternative wouldinvolve an official agreement between operators to allow internetworkreference signalling. A data store 298 in the channel estimation unit295 stores a weight matrix for beamforming when providing a wirelesscommunication link between a user equipment and a plurality of aerialvehicles that includes weights determined responsive to the determinedpropagation characteristic of each further wireless communicationchannel.

Certain embodiments of the present invention thus provide a process forestimating a wireless channel attenuation and phase shift betweenmultiple user devices and multiple aerial antennas mounted on movingHAPs. This channel information is used to provide mobile coverage to theuser devices. The process can be used to provide mobile coverage tomultiple user devices in the same frequency-time spectrum optionallyutilising equalisation and precoding techniques.

User scheduling can optionally be utilised where users are divided intooverlapping clusters of concurrent transmissions and every cluster ofuser is assigned a proportion of frequency-time resources. Certainembodiments of the present invention augment concurrent usercommunication clusters solely for the purpose of interferencecancellation and not for data communication.

Certain embodiments of the present invention relate to the processes andrequired apparatus for providing mobile coverage to multiple userdevices using two or more aerial antennas mounted on HAPs. A precisechannel estimation procedure can be utilised that detects theattenuation and phase shift between every user device and every aerialantenna on a HAP. Simultaneous communication can then occur withmultiple user devices using the estimated channel information.

Certain embodiments have been described which estimate channels betweenuser equipment and HAPs. Reference has been made to determining anamplitude and phase for each channel. This can be determined in variousways according to certain embodiments of the present invention. A way isfor a user equipment to measure time difference of arrival of referencesignals (in time units, e.g. increments of a special basic LTE timeunit) and send them back. It will be appreciated that for otherimplementations the same information may be expressed differently. Thatis to say different time units or expressed as phase. The phase for acomplex number can be derived from the delay according to the equation:θ=−2πf(τ−τ₀)  (10)

Where θ is the phase in radians, f is the carrier frequency in Hz, and(τ−τ₀) is the measured time difference of arrival of a reference signalin seconds, relative to time τ₀, e.g. the arrival time of the firstreceived reference signal.

FIG. 11 is a view of certain nodes in the communication network 296. Afirst node 500 is provided by the user equipment in the communicationnetwork 296. A single user equipment is illustrated in FIG. 11 . Afurther node 510 is the HAP-Cell system which includes the cellsgenerated by the constellation of HAPs 200, the HAPs themselves and theHAP-Cell processing centre 265. A further node 1100 represents the corenetwork which is connectable to an internet 1110 such as the World WideWeb or the like. The core network 1100 includes a mobility managemententity (MME) 1120 which communicates with the processing centre viacontrol plane data paths. The core network 1100 also includes a servinggateway (S-GW) 1130 which communicates with the processing centre 265 ofthe HAP-Cell system via a user plane connection. The core networklikewise also includes a packet data network gateway (PDN-GW) 1140 whichcommunicates with the serving gateway 1130 and the internet 1110. FIG.11 thus helps demonstrate the control plane and user plane connectionsbetween different entities in a cellular network. Control signallingbetween an access network and core network is performed via the mobilitymanagement entity 1120. Thus centralised control of effective basestations is controlled via the mobility management entity. This entityalso helps ensure the connectivity of the virtual base stations to theinternet by controlling user data paths from a given effective basestation to a particular serving gateway 1130 which in turn is connectedto a particular packet data network gateway 1140.

FIG. 11 thus helps provide a top level illustration of a mobile networkthat involves a constellation 200 of high altitude platforms that createcells on the ground via highly directional antennas mounted on them. Theconstellation 200 of HAPs can behave as an effective set of remote radioheads (RRHs). These perform beamforming and transmit/receive wirelesssignals to/from user equipment in the communication network. Aprocessing centre 265 controls operation of the HAPs i.e. sendsbeamforming instructions and relays cellular network signals that takeplace between user the equipments and the HAPs. The processing centre265 also helps associate virtual base stations with the beams created bythe HAPs. Thus making the HAP-Cell system appear like a conventionalcellular access network as far as standard user equipment is concerned.It is to be noted that in order to help integrate with otherconventional cellular architecture the HAP-Cell system can optionally beconfigured to appear to the core network 1100 like a standard cellularaccess network. To this end the HAP-Cell processing centre has a controlplane interface with the mobility management entity 1120 and a user datachannel to the serving gateway 1130. Virtual base station base bandunits maintained at the processing centre 265 will thus haveconnectivity with an operator's core network and therefore with the restof the internet 1110.

FIG. 12 helps illustrate how an initial user acquisition can occur usingdownlink channel estimation via a single HAP wide cell. That is to sayFIG. 12 helps illustrate how user equipment can acquire a communicationlink to a core network first using a wide cell coverage area. This isprovided by a respective single channel wireless communication link.User acquisition via a wide cell coverage area initially is advantageoussince user acquisition via a narrow cell formed by a multi-channelwireless communication link makes it otherwise difficult to locate userequipment in an initial phase. FIG. 12 helps illustrate a first phase1205 referred to as user registration. FIG. 12 helps illustrate a secondphase 1210 referred to as initial channel estimation via a single HAPwide cell. FIG. 12 helps illustrate a third phase 1215 in which ahandover step occurs from a single wide cell provided by a single HAP toa P-cell provided by multiple HAPs.

In the first phase 1205 a user equipment receives beacon signals fromthe various HAPs in a relevant geographical location. Via an associationrequest step S 1220 the user equipment makes an association request viathe strongest wide coverage area cell associated with the HAP having thestrongest beacon signals. Via an authentication step S 1225 userequipment authentication and registration signalling occurs between theHAP-Cell system node 510 and the core network node 1100. After this theHAP-Cell system node 510 sends an effective association response via thewide coverage cell selected by the user equipment. This associationresponse is illustrated by a response step S 1230. This is followed byan instruction from the HAP-Cell system node 510 to instruct the userequipment to measure time difference of arrival (TDoA) of beacon signalsfrom other cells. This instruction is illustrated by an instruction stepS 1235. This step marks the start of the second phase of the useracquisition process. Via a report step S 1240 the user equipment sendsback a measurement report.

In response to the measurement report provided by the user equipment atstep S 1240 the HAP-Cell system node 510 can determine the userequipment's position based on the TDoA of beacons from other cells. Thisis illustrated by a position determining step S 1245. Thus FIG. 12 helpsshow a way of determining a user location that can make use of certainstandardised dard measurements performed by user equipment. Observedtime difference of arrival measurements can be utilised. A userequipment is initially connected to a wide coverage cell e.g. created byan aerial antenna on a single HAP using a standard cell associatedprocedure. In order to help boost a range of the wide coverage, low gaincell beacon signal if necessary, the HAP can utilise a low band widthchannel e.g. a 1.4 MHz or 3 MHz LTE channel. Once a user equipment isacquired by a wide coverage cell it can be instructed to measure theOTDoA of beacon signals from other HAPs. The other HAPs in theconstellation will direct their beams towards the same cell area andtransmit beacons with unique physical cell ID numbers detectable by theuser equipment. When the user equipment sends back the OTDoA measurementreport back to a serving effective base station i.e. the wide coveragecell, the HAP-Cell processing centre will be able to see thesemeasurements in conjunction with its accurate knowledge of the HAPlocations. This combined information can be utilised to determine a userequipment location.

Accuracy of the OTDoA measurement based approach can be limited by thequantisation error of standard user equipment devices. For example inthe case of LTE the step size is approximately 32 nanoseconds i.e. abasic LTE time unit equal to 1/(15000×2048) seconds.

Nevertheless according to certain embodiments of the present inventionthis approach can be used as a first step in determining the userequipment location to within 1 to 10 m of accuracy.

Subsequent to the determination of a user equipment position at step S1245 that location is sent to the core network illustrated via a storagecontrol step S 1250 where a respective location is stored for each userequipment. Subsequent to the location of the user equipment beingaccurately determined it is possible to use multiple HAPs to create anarrow cooperative beam at the detected user equipment location. This isillustrated by a beam creation step S 1255 in FIG. 12 and begins thestart of the third phase 1215 in the user acquisition process. This newnarrow beam is registered as a cell via a registration step S 1260.Subsequent to the registration of this narrow beam as providing aneffective narrow cell and thus an effective base station, a handovercommand/request can be exchanged between the user equipment and theHAP-Cell system. This is illustrated by a handover command step S 1265.Association and synchronisation thereafter takes place between the userequipment and the P-cell created via cooperative beamforming andillustrated by an association and synchronisation step S 1270 in FIG. 12. After association and synchronisation with the user equipment and theP-cell a request is transferred from the HAP-Cell system node 510 to thecore network node 1100 to request user plane path switching for thatparticular user equipment. This is illustrated by request step S 1275 inFIG. 12 . An update step S 1280 illustrates the update of paths for thatparticular user equipment subsequent to the request for a user planepath switch. This results in a path switch acknowledgement being sent aspart of an acknowledgment step S 1285 from the core network to the cellprocessing centre in the HAP-Cell system which results in the userequipment being in communication with the core network via themulti-channel wireless communication link associated with the P-cell.

FIG. 13 illustrates an alternative to user acquisition using an uplinkchannel estimation via an initial single HAP wide cell. The useracquisition process includes three phases. A first phase 1305 is a userregistration phase. A second phase 1310 is an initial channel estimationvia single HAP wide cell phase. A third phase 1315 is a handover phasein which a user handover occurs from the wide coverage cell provided bya single HAP to a narrow coverage cell provided via a multi-channelcommunication link from multiple HAPs.

An association request step S 1320 illustrates an association requestbeing sent from a user equipment to the HAP-Cell system. This is inresponse to beacon signals being transmitted from the HAPs in a relevantgeographical area. The association request is made to associate with awide coverage area cell provided by the HAP which provides the strongestbeacon signal. Subsequent to this request an authentication step S 1325occurs whereby user equipment authentication and registration takesplace between the HAP-Cell system and the core network. Thereafter anassociation response is sent as a response step S 1330 via the widecoverage cell originally selected by the user equipment. After thisassociation response is sent the HAP-Cell system instructs the userequipment to transmit a wireless signal. Aptly this is an uplinkreference signal (URS). This is illustrated by an instruction step S1335. This is the start of the second phase of the user acquisitionprocess. Next the user equipment transmits URS via a transmit step S1340. Responsive to the received URS signals, as illustrated by step S1345 in FIG. 13 , it is possible to determine a user equipment'sposition based on the phase difference or URS received at multiple HAPs.FIG. 13 thus illustrates an alternative approach to the OTDoApositioning method for accurately determining a location of userequipment. In this process illustrated in FIG. 13 the user equipmentposition is determined by the HAP rather than the user equipment. Thewide coverage area cell to which the user equipment is initiallyconnected instructs the user equipment to transmit uplink referencesignals (URS). Aptly this can be a sounding reference signal (SRS). Theuser equipment is thus told to transmit sounding signals by a basestation to begin a P-cell user acquisition. Optionally a wide cell or anarray of P-cells can be utilised to look for the user equipment. Aptlythe URS is used for uplink channel estimations. In this way aerialantennas mounted on HAPs work as receivers measuring the phasedifference of the URS arriving from the user equipment. An advantage ofthis approach is that it is not limited by any quantisation errorimposed by a standard user equipment device but rather by an accuracy ofmeasurements which is determined by the proprietary nature of theHAP-Cell system. Using this technique a user equipment device locationcan be determined to within 0.5 m accuracy. Even greater degrees ofaccuracy can be achieved according to certain embodiments of the presentinvention.

Once the location of the user equipment is determined the HAP-Cellsystem provides this location to the core network and the user equipmentlocation is stored in the core network. This is illustrated by a storagecontrol step S 1350 in FIG. 13 . Once the user equipment location isknown it is possible to utilise multiple HAPs to create a narrowcooperative beam at the detected user equipment location. This isillustrated by a beam creation step S 1355 in FIG. 13 which is theinitial step in the third phase of the user acquisition process.Thereafter the narrow beam is registered as a cell via a registrationrequest illustrated by a registration step S 1360. A handovercommand/request is then carried out via the original wide coverage cellto which the user equipment is at that stage still connected. This isillustrated by a handover command step S 1365 in FIG. 13 . Thereafterassociation and synchronisation occurs with the new narrow beam P-cell.This is illustrated by step S 1370 in FIG. 13 . Thereafter a user planepath switch is requested for the user equipment from the HAP-Cell systemto the core network. This is illustrated by step S 1375. After this,paths for the user equipment are updated. This is illustrated by step S1380. Thereafter a path switch acknowledgment is sent from the corenetwork to the HAP-Cell system via step S 1385 and communication occursbetween the user equipment and the core network via a multi-channelcommunication link provided by a constellation of HAPs. Thus aftercreating an ultra-narrow cooperative beam at a detected user equipmentlocation the HAP-Cell processing centre can register the P-cell as a newcell. I.e. the resulting newly spawned virtual base station can beassociated with the core network. Once the new ultra-narrow beam isregistered as a standard cell the user equipment can start receivingbeacon signals with its physical cell ID at a significantly higher powerthan its currently serving wide coverage area. This triggers the userequipment to associate with the narrow cell by appearing as the bestcell in terms of signal strength. Afterwards, depending upon a userequipment state (i.e. idle or actively uploading/downloading data),either the user equipment will send a handover request to the wide areacell base station or the base station will send a handover command tothe user equipment. Once a user equipment is associated and synchronisedwith its cooperative inter-HAP beam P-cell, the HAP-Cell processingcentre will use its interface with the core network to update the userplane data paths. This helps make sure the user equipment and the P-cellbase station are connected to appropriate gateways and thus haveconnectivity to the internet. FIG. 14 illustrates certain elements ofthe communication network 296 shown in FIG. 2 in more detail in terms oftheir use during the acquisition process. That is to say FIG. 14illustrates user acquisition apparatus. A constellation 200 of HAPs cancreate a multi-channel wireless communication link to a user equipmentthus providing a P-cell at the location of the user equipment. One ormore of the HAPs in the constellation likewise provides a single channelwireless communication link which provides a wide coverage cell at aregion which includes where the user equipment is located. Theconstellation of HAPs communicates to the ground station 255 which isconnected to the HAP-Cell processing centre 265. FIG. 14 illustratescertain elements in the HAP-Cell processing centre such as thebeamforming controller 275. Also illustrated is a virtual wide coveragecell controller 1400 and a virtual P-cell controller 1410. Thebeamforming controller 275 and wide and narrow cell controllers 1400,1410 communicate with a user acquisition controller 1420. The useracquisition controller 1420 is connected to the beamforming controller275 and provides beamforming instructions to the beamforming controllerwhich then communicates via a control interface and a connection 270 tothe ground station or ground stations and from there to theconstellation of HAPs via a HAP-Cell control interface. This isassociated with a cellular fronthaul interface.

The user acquisition controller 1420 likewise exchanges control signalsbetween the virtual wide coverage cells controller 1400 which determinesoperation of virtual wide coverage cells and the virtual P-cellscontroller 1410 which determines operation of virtual P-cells. Controlsignals are exchanged between associated connections as are handoverinstructions. The virtual wide coverage cell controller 1400 and virtualP-cell controller 1410 communicate with the ground station/s 225 via acellular fronthaul interface and a connection 270.

Thus certain embodiments of the present invention provide a method forconnecting a mobile user equipment to a core network. The method caninclude the steps of providing a single channel communication linkbetween a user equipment and one of a plurality of aerial vehicles. Viathe plurality of aerial vehicles a multi-channel communication link isprovided with the user equipment. Thereafter, via a handover step,communication between the user equipment and the core network istransferred from communication via the single channel communication linkto communication via the multi-channel communication link. The step ofproviding a single channel communication link includes forming a beam,from a directional antenna of an aerial vehicle, towards the ground thatilluminates a first cell coverage area this is a so-called wide cellcoverage area. The cell coverage area of a wide coverage cell has afootprint with a width of greater than 500 m.

The step of providing a multi-channel communication link comprisescooperatively forming a beam, via respective directional antennas of theplurality of aerial vehicles, towards the ground that illuminates afurther cell coverage area. This further cell coverage area is muchnarrower and may be referred to as a P-cell. The further cell coveragearea has a footprint with a width of less than 1 m. Optionally thefootprint has a width of less than 0.5 m. In this sense a footprint of acell coverage area comprises a region where a wireless signal strengthis strong enough that a user equipment located within the cell coveragearea can associate with the core network via an associated singlechannel communication link or a multi-channel communication link. Themethod of user acquisition includes the step of determining a locationof the user equipment responsive to a wireless signal exchanged via asingle channel wireless communication link. Communication between theuser equipment and a core network is thereafter transferred selectivelyresponsive to the location of the user equipment being determined.Various optional techniques can be utilised for determining thelocation. One of these techniques utilises measurement of OTDoA ofbeacon signals. Another option is to instruct user equipment to transmitan uplink reference signal such as a sounding reference signal. Otheroptions could of course be utilised according to certain embodiments ofthe present invention.

FIGS. 15 and 16 illustrate user equipment tracking subsequent to useracquisition of user equipment with a narrow P-cell provided by amulti-channel wireless communication link. As the user equipment moves,which may be likely if the user equipment is a mobile device such as asmart phone, knowledge of a location of the user equipment is useful tohelp during the multi-channel beamforming process. Likewise the locationis useful in determining if a multi-channel wireless communication linkis likely to be dropped. As a user equipment moves the determination ofits location will enable the HAP-Cell processing centre to dynamicallyadjust amplitude and phase weights for each HAP involved in anycooperative beamforming.

FIG. 15 illustrates a first approach that can be employed to track auser equipment location. This can make use of certain conventionalcellular signalling schemes but assumes that the user equipment isalready connected to a P-cell (as above described). As illustrated inFIG. 15 several additional P-cells can be created to surround a servingP-cell beam. Each of these additional P-cells transmit beacons with itsown unique cell ID. Such beacons with cell IDs are used by cells. Theuser equipment is then instructed by its serving P-cell to report backthe power level of the cell beacons it receives. FIG. 15 thusillustrates a constellation 200 of HAPs which create multiplemulti-channel wireless communication links via cooperative beamformingtechniques. Five multi-channel wireless communication links 1500 ₁₋₅ areshown in FIG. 15 . Each of these multi-channel wireless communicationlinks provides a respective P-cell 1510 ₁₋₅. One P-cell 1510 ₃ is shownfocused on the specific location where a user equipment is determined tobe located. This is the serving cell. The four further P-cells aredirected to locations proximate and to some extent surrounding theserving P-cell. It will be appreciated that two, three, four, five ormore P-cells can be utilised at locations close to the servicing P-cell.The P-cells may overlap to some extent or optionally may not overlap.Aptly the P-cells that are auxiliary or extra P-cells overlap by 10% ormore with each other and/or the serving P-cell.

Beacon signals are transmitted (illustrated by arrow 1520 in FIG. 15 )on the downlink to the user equipment which then sends a measurementreport 1530. The beacons with cell IDs are used by cells. The userequipment is instructed by its serving P-cell to report back the powerlevels of the cell beacons it receives. Given this information theprocessing centre can estimate in which direction the serving P-cellbeam has to be moved to track the user. E.g. in the direction of thestrongest surrounding P-cell beacon.

FIG. 16 illustrates an alternative technique by which it is possible totrack movement of user equipment. This helps a processing centreestimate in which direction a serving P-cell beam has to be moved tokeep track with the user. This second approach is based on uplinkreference signalling (URS). A serving P-cell indicated as beinggenerated by a respective multi-channel wireless communication link 1600generates an associated P-cell 1610 at a location of the user equipment.The serving P-cell requests the user equipment to transmit frequent URS.The instruction is illustrated by an arrow 1620 in FIG. 16 . In responsethe user equipment transmits the uplink reference signals illustrated bya respective arrow 1630 in FIG. 16 . Which are then received by everyHAP in the constellation 200 individually. The received signals can besubsequently analysed by the processing centre to determine the userequipment location. For example, the user equipment can be requested totransmit URS as frequently as every 2 ms. This will be sufficient totrack highly mobile user equipment e.g. those travelling in cars or evenvia high speed trains.

FIG. 17 illustrates a situation which may arise in a HAP-Cell systemwith cooperative interplatform beamforming. Such a situation, or risk,is a loss of connectivity with the user equipment with a narrow P-cell.This may occur when the user equipment walks into a building thuspotentially introducing a significant change in the radio propagationpaths to the HAPs. Alternatively this may occur for a very fast movinguser equipment in certain geographical territories or the like. For suchcases certain embodiments of the present invention can implement anintelligent detection mechanism which uses user equipment locationtracking information and other control signals to help predict when sucha loss of P-cell connectivity is about to occur. When this is determineda handover process can be initiated from the relatively narrow P-cellback to a wider wide coverage cell associated with a single channelwireless communication link. In such a way it can be possible tomaintain a user equipment's continuous connection to a network. This canbe utilised in situations when a dedicated P-cell becomes out of rangewith user equipment. It will be appreciated that optionally certainembodiments of the present invention can thereafter carry out a furtherhandover switch from the wide coverage cell to a narrow coverage cellwhen it is determined that a user equipment location and movement issuch that a P-cell can be utilised.

FIG. 17 illustrates three respective nodes of a communication network. Afirst node 500 is associated with user equipment. A further intermediatenode 510 is associated with a HAP-Cell system. A third node 1100 isassociated with a core network of the communication network. Asillustrated in FIG. 17 via a prediction step S 1700 the HAP-Cell systempredicts that the user equipment is likely to lose connectivity with aserving P-cell. This prediction can occur via different techniquesaccording to certain embodiments of the present invention. If it isdetermined that the user equipment is moving in a manner likely to loseconnectivity a handover command step S 1710 occurs via a command beingpassed from the HAP-Cell system node 510 to the user equipment.Thereafter an association and synchronisation step S 1720 seesassociation and synchronisation signals being exchanged with a widecoverage cell. Subsequently a request step S 1730 sees a request for auser plane path switch for the particular user equipment being sent tothe core network. An update step S 1740 occurs within the core networkwhereby the paths for communication etc for the user equipment areupdated. Once this has been carried out a path switch acknowledgment issent from the core network to the HAP-Cell system as indicated by theacknowledgement sending step S 1750 in FIG. 17 .

Certain embodiments of the present invention thus provide a method formaintaining a connection of user equipment which is mobile to a corenetwork. The method includes the steps of providing a multi-channelcommunication link between a user equipment at a first location and aplurality of aerial vehicles. Next a determination step determines thatthe user equipment is moving whereby a loss of connectivity via themulti-channel communication link may occur. Subsequently, via a handoverstep, communication between the user equipment and the core network canbe transferred from communication via the multi-channel communicationlink associated with the initial serving P-cell to communication via asingle channel communication link provided by a respective single aerialvehicle. The single channel communication link can be provided byforming a beam, from a directional antenna of a respective aerialvehicle, towards the ground that illuminates a first cell coverage area.The first cell coverage area has a footprint with a width of greaterthan 500 m. The step of providing the multi-channel communication linkcomprises cooperatively forming a beam, via respective directionalantennas of the plurality of aerial vehicles, towards the ground thatilluminates a further cell coverage area. The further cell coverage areahas a footprint with a width of less than 1 m and optionally less than0.5 m.

Determining that a user equipment is moving can be carried out indifferent ways. For example, the step of determining that the userequipment is moving can comprise providing additional P-cellsgeographically proximate to a cell coverage area provided by a servingP-cell and transmitting respective beacon signals via those additionalwireless communication links. Determining motion can occur responsive toa power level of beacon signals received at the user equipment from theadditional cells. Determining that connectivity is likely to be lost canbe determined in various ways such as by determining that a speed ofmovement exceeds a predetermined threshold value. Alternatively, withknowledge of a particular geographical location which can be storedseparately a determination can be made that a user equipment is movinginto a zone whereby loss of connectivity with a serving P-cell islikely. An alternative process for determining that user equipment ismoving can include instructing a user equipment to transmit an uplinkreference signal and receiving the transmitted signals at multiplepoints.

FIG. 18 illustrates user equipment location tracking apparatus. Inparticular FIG. 18 illustrates certain elements of a communicationnetwork previously described which focus on user equipment tracking. Asillustrated in FIG. 18 a constellation 200 of HAPs can exchange wirelesssignals with a coverage area which can include multiple user equipment.The constellation 200 of HAPs is in wireless communication with one ormore ground station/s 255 which communicate with the constellation ofHAPs via a HAP-Cell control interface and provides an effective cellularfronthaul interface. The HAP-Cell processing centre 265 includes achannel estimation controller 295, a virtual wide coverage cellcontroller 1800 and a virtual P-cell controller 1810. A user trackingcontroller 1820 in the HAP-Cell processing 265 receives channelestimation information from the channel estimation unit 295. The userequipment tracking controller sends signalling instructions to thevirtual wide coverage cell controller 1800 and signalling instructionsto the virtual P-cells controller 1810. These exchange uplink/downlinkcontrol signals via the ground station/s 255. The channel estimationunit 295 exchanges information with the ground station/s 255 via aHAP-Cell control interface. The user equipment tracking controller atthe HAP-Cell processing centre stores estimated locations of all userequipment connected to the HAP-Cell system. This includes recenttrajectories of their mobility. This helps enable the prediction oftheir location during upcoming packet transmissions. The user equipmenttracking controller has an interface with the virtual cells serving theusers through which it will instruct the cells to transmit requiredcontrol and reference signals. This can occur at a specified frequencyboth on an uplink and downlink. The interface between the user equipmenttracking controller and the channel estimation unit enables the userequipment tracking controller to obtain attenuation and phase differenceof reference signals, received at or transmitted from multiple HAPs.This helps determine a user's location.

FIG. 19 helps illustrate an approach that can help further facilitatethe HAP-Cells systems ability to avoid loss of P-cell connectivity. Thiscan be provided to help enable the user equipment to be simultaneouslyconnected with a wide coverage cell and, when possible or at selectedtimes, the user equipment can also be connected to a narrow P-cell. Thisis a multipoint scheme where multiple cells can be used by a single userequipment simultaneously. The user equipment will communicate all datatraffic via a P-cell whilst it is in reach. A minimum required controlplane signal exchange occurs with a wide coverage cell. In the event oflosing connectivity to the P-cell the user equipment can be instructedto redirect all of its data traffic via the wide coverage cell thus notdropping the connection.

FIG. 20 schematically illustrates an alternative way of useracquisition, prior to having a registered relationship with a userequipment and a P-cell and prior to tracking location. A HAP-Cell systemcreates many narrow beams and transmits respective beacon signals viathose beams thus presenting each as a “standard cell”. This is on thebasis/hope that one or more of the beacon signals may “hit” a userequipment. Under such circumstances a user equipment in the catchmentarea of the generated narrow beams responds with an association requestusing a synchronisation sequence it receives as part of the beacon. As aresult, a narrow P-cell is already provided by optimistically/blindlyhitting a user equipment with it.

FIG. 20 thus illustrates three respective nodes of a communicationnetwork. A first node 500 is associated with the user equipment. Afurther intermediate node 510 is associated with a HAP-Cell system. Athird node 1100 is associated with a core network of the communicationnetwork. As illustrated in FIG. 20 via a beacon signal transmission stepS 2000 the HAP-Cell system generates a plurality of narrow beam P-cellsand transmits beacon signals with respective cell IDs for each of thosenarrow beam P-cells. The direction in which the narrow beams are pointedis random or predetermined across a service area associated with theHAP-Cell system. Alternatively the beams are transmitted in directionspredicted to be where user equipment may be located. Some of thesegenerated narrow beams will be directed to areas where user equipment islocated. For these, the user equipment will send an association requestto the HAP-Cell system. The associated request is illustrated via step S2010 in FIG. 20 . Subsequent to the association request the HAP-Cellsystem node 510 exchanges signals with the core network node 1100 aspart of an authentication and registration step S 2020. Subsequently anassociation response S 2030 is sent to the user equipment. Thereafterthe user equipment's position is determined S 2040 based on a beamdirection of the selected P-cell. This location is stored as respectiveuser equipment location information in the core network and thereafterchannel estimation can occur as per previously described.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of them mean “including but notlimited to” and they are not intended to (and do not) exclude othermoieties, additives, components, integers or steps. Throughout thedescription and claims of this specification, the singular encompassesthe plural unless the context otherwise requires. In particular, wherethe indefinite article is used, the specification is to be understood ascontemplating plurality as well as singularity, unless the contextrequires otherwise.

Features, integers, characteristics or groups described in conjunctionwith a particular aspect, embodiment or example of the invention are tobe understood to be applicable to any other aspect, embodiment orexample described herein unless incompatible therewith. All of thefeatures disclosed in this specification (including any accompanyingclaims, abstract and drawings), and/or all of the steps of any method orprocess so disclosed, may be combined in any combination, exceptcombinations where at least some of the features and/or steps aremutually exclusive. The invention is not restricted to any details ofany foregoing embodiments. The invention extends to any novel one, ornovel combination, of the features disclosed in this specification(including any accompanying claims, abstract and drawings), or to anynovel one, or any novel combination, of the steps of any method orprocess so disclosed.

The reader's attention is directed to all papers and documents which arefiled concurrently with or previous to this specification in connectionwith this application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

The invention claimed is:
 1. A communication network, comprising: aplurality of aerial vehicles that each supports at least one respectivedirectional antenna; and a processing element for determining at leastone propagation characteristic for each respective wirelesscommunication channel between at least one user equipment and eachaerial vehicle of the plurality of aerial vehicles; wherein theprocessing element determines the propagation characteristic, for eachwireless communication channel, via comparing an amplitude and phase ofa received wireless signal transmitted via the respective wirelesscommunication channel with a predetermined reference amplitude andreference phase.
 2. The network as claimed in claim 1, furthercomprising: a ground based cell processing centre that comprises theprocessing element and includes at least one interface to a core networkand optionally includes an aerial vehicle flight control unit, a channelestimation unit and a beamforming control unit; and at least one groundstation, each comprising a directional antenna element, arranged torelay user data and control information between each aerial vehicle andthe cell processing centre.
 3. The network as claimed in claim 1,wherein the at least one propagation characteristic comprises anestimated gain and an estimated phase shift of the wirelesscommunication channel.
 4. The network as claimed in claim 1, whereineach user equipment is arranged to transmit a reference signal from theuser equipment to each of the aerial vehicles.
 5. The network as claimedin claim 1, wherein each aerial vehicle is arranged to transmit areference signal from the aerial vehicle to a user equipment.
 6. Thenetwork as claimed in claim 1, further comprising: the directionalantenna of each aerial vehicle comprises at least one multi elementdirectional antenna array.
 7. The network as claimed in claim 1, furthercomprising: a data store that stores a channel matrix for wirelesscommunication channels between every aerial vehicle and every userequipment, said channel matrix comprising a complex number for eachwireless communication channel for each high altitude platform-userequipment (HAP-user equipment) pair.
 8. The network as claimed in claim1, further comprising: at least one multi-channel wireless communicationlink each provided between each respective user equipment and theplurality of aerial vehicles.
 9. The network as claimed in claim 8,further comprising: when a total number of user equipment exceeds apredetermined number, each user equipment is allocated a less than 100%share of a basic airtime bandwidth resource of a respectivemulti-channel wireless communication link associated with that userequipment and the plurality of aerial vehicles.
 10. The network asclaimed in claim 1, further comprising: at least one further wirelessdevice that is not included in a communication with the aerial vehiclesvia a multi-channel wireless communication link.
 11. The network asclaimed in claim 10, further comprising: the processing element isarranged for determining at least one propagation characteristic foreach respective further wireless communication channel between eachfurther wireless device and each aerial vehicle of the plurality ofaerial vehicles.
 12. A method of determining at least one propagationcharacteristic of each wireless communication channel between at leastone user equipment and a plurality of aerial vehicles, comprising thesteps of: for each respective wireless communication channel betweeneach of at least one user equipment and each aerial vehicle of aplurality of aerial vehicles, determining an amplitude and phase of awireless signal transmitted between the user equipment and an aerialvehicle associated with the respective wireless communication channel;and for each wireless communication channel, determining at least onepropagation characteristic of the wireless channel by comparing thedetermined amplitude and phase with a corresponding predeterminedreference amplitude and reference phase.
 13. The method as claimed inclaim 12, further comprising: determining the at least one propagationcharacteristic comprises determining an estimated gain and an estimatedphase shift of the wireless communication channel.
 14. The method asclaimed in claim 12, further comprising: for each wireless communicationchannel, transmitting a reference signal that comprises the wirelesssignal from the respective user equipment to the respective aerialvehicle and determining the amplitude and phase of the receivedreference signal responsive thereto.
 15. The method as claimed in claim12, further comprising: for each wireless communication channel,transmitting a reference signal that comprises the wireless signal fromthe aerial vehicle associated with the wireless communication channel tothe user equipment associated with the wireless communication channel;determining an amplitude and phase of the reference signal received atthe user equipment; and transmitting a measurement report indicating thedetermined amplitude and phase from the user equipment to the aerialvehicle.
 16. The method as claimed in claim 12, further comprising:providing at least one wireless communication link between each userequipment and the plurality of aerial vehicles via a cooperativebeamforming method responsive to said determined propagationcharacteristics.
 17. A method of wireless communication between at leastone user equipment and a plurality of aerial vehicles, comprising thesteps of: for each respective wireless communication channel between atleast one user equipment and each aerial vehicle of a plurality ofaerial vehicles, determining an amplitude and phase of a wireless signaltransmitted between the user equipment and an aerial vehicle associatedwith the respective wireless communication channel; for each wirelesscommunication channel, determining at least one propagationcharacteristic of the wireless communication channel by comparing thedetermined amplitude and phase with a corresponding predeterminedreference amplitude and reference phase; for each wireless communicationchannel, determining, based on the at least one propagationcharacteristic, at least one weighting to be applied to signalstransmitted and/or received via a wireless communication link betweenthe user equipment and the aerial vehicles; and providing a wirelesscommunication link between the user equipment and the plurality ofaerial vehicles responsive to the determined at least one weighting. 18.The method as claimed in claim 17, further comprising: applying arespective weighting to wireless signals transmitted from a transmittingaerial vehicle, via the further wireless communication link, therebycausing the transmitted signals to arrive at the user equipmentsubstantially in phase with corresponding wireless signals arriving atthe user equipment from at least one further aerial vehicle.
 19. Themethod as claimed in claim 17, further comprising: providing thewireless communication link via a cooperative beamforming methodperformed by the plurality of aerial vehicles.